This invention relates in general to signal filtering to separate a signal of interest from a noisy background and relates more particularly to endpoint detection in a wafer fabrication process that has a periodic or approximately periodic (i.e., semiperiodic) temporal variation of a spatial distribution of input light utilized to detect endpoint. In the following discussion, "radiation" means any incident flux of particles or waves and in particular includes the UV/Visible range of wavelengths typically used in optical emissions spectroscopy. "Semiperiodic" means an approximately periodic signal that is the result of a substantially periodic process that produces a substantially periodic signal and an associated, resultant signal that may be aperiodic.
It is becoming increasingly important to be able to accurately detect the endpoint of various steps in a wafer fabrication process. Submicron linewidths and ultrathin layer thickness are becoming common, especially in state of the art devices. In such devices, overprocessing can undercut features, thereby severely affecting yield.
Also, these state of the art devices require processing in single wafer systems. To maintain the same throughput as conventional batch wafer process systems, these single wafer systems must complete each process step much faster than was required in the batch process systems. It is therefore important for system throughput to be able to stop a process step as soon as it is completed. Although it was acceptable to run a batch wafer process system for a preselected interval, in single wafer systems it is important to be able to accurately detect the endpoint of a process step so that processing can be quickly terminated.
In virtually all dry etch processes, such as plasma etch, reactive ion etch (RIE), ion milling, reactive ion beam etch (RIBE), and magnetron etching, light is emitted from the gas phase reactants, from the gas phase products and/or from the film being etched. Etch endpoint occurs when the exposed portion of a film being etched has been completely etched from the substrate surface.
At etch endpoint, some product species cease to be produced and some reactant species cease to be consumed. Therefore, in the gas phase, the reactant species quickly increase in concentration and the product species quickly decrease in concentration. These changes produce concomitant changes in the associated emission and/or absorption spectra intensities of the gas phase and film. Various endpoint detectors are designed to detect this change in optical intensity. Similar changes occur in the optical output spectra of other types of wafer processing systems.
In the endpoint detection system of FIG. 1, light is passed through a fiber optic cable 10 from a wafer processing system, such as the Applied Materials PE5000 plasma chamber 11, to a spectral detector such as monochromator system 12. This system includes a monochromator 13 that is adjusted to direct onto a photomultiplier tube 14 light of a wavelength that changes in intensity at the end of a process step. A high voltage power supply 15 applies to the photomultiplier tube a voltage that can be adjusted to vary the gain of the photomultiplier tube.
A motor 16 is connected by a drive shaft 17 to a radiation dispersive element in the monochromator. This enables the dispersive element to be rotated to change the wavelength component that is directed onto the photomultiplier tube. A scanner interface board 18 connects elements 14, 15 and 16 to an endpoint system 19 that selects the applied voltage and detected wavelength and that receives an input signal S from photomultiplier tube 14. Endpoint system 19 also processes the input signal S from the monochromator to extract in real time a portion of this signal that is indicative of the endpoint of a process step. In an etch process, this endpoint system detects when the etch process reaches and clears a layer interface in the wafer, thereby defining the end-of-etch condition.
A spectrophotometer can be utilized in place of monochromator 13 and photomultiplier tube 14 to provide to endpoint system 19 a spectral output. In such a system, endpoint system 19 would extract from this spectral output the amplitude of the spectral distribution of the chemical species that is being monitored for endpoint detection. Such a system has the advantage of utilizing more than one frequency component of light, thereby improving the signal to noise ratio of the light from the chemical component being monitored.
FIGS. 2 and 3 illustrate the approximately periodic (i.e., semiperiodic) temporal variation of the input signal S to endpoint system 19. Although the rotation of a magnetic field in reactor chamber 11 is periodic, the variation in light intensity to the detector is not purely periodic, but instead is only semiperiodic. By "semiperiodic" is meant the periodic component as well as the associated nonlinear fluctuations that are produced by this periodic variation of a system parameter. FIG. 2 illustrates this signal on a time scale illustrating about 5 semiperiodic cycles. FIG. 3 illustrates this signal on a time scale containing about 40 cycles. FIG. 4 illustrates the temporal variation of the concentration of the chemical component being monitored for endpoint detection. These figures illustrate that the variation of the input signal S is about 15 times larger than the signal of interest in endpoint detection.
In this system, the input signal S to endpoint system 19 is semiperiodic with a period of about 2 seconds. Most of this semiperiodic variation can be removed by averaging over a sufficient number of cycles or by passing this signal through a low pass filter to remove this sinusoidal variation. Typically, a running average of the most recent 20 cycles is produced to decrease the sinusoidal component by a factor of about 20. Since the initial ratio of semiperiodic variation of FIG. 3 compared to the variation of interest of FIG. 4 is about 15, this duration of averaging reduces the semiperiodic component to an acceptable level.
The criterion for endpoint detection is that the ratio of the slope of the averaged signal to a reference slope exceeds 1. If the chemical monitored for endpoint detection increases in concentration at the endpoint, then this reference slope is positive and if the chemical monitored for endpoint detection decreases in concentration at the endpoint, then this reference slope is negative.
Because the semiperiodic component of the averaged signal is still significant, a box method is utilized to eliminate false detections of endpoint. In accordance with this method, the amplitude of the averaged signal is measured periodically at fixed intervals I (e.g., every second) and the incremental change in this amplitude is divided by the temporal interval between these successive measurements times the reference slope. If this ratio .alpha. exceeds 1, then the measurement and calculation are repeated at a time equal to I/.alpha.. If the new value .alpha.' again exceeds 1, then the measurement is repeated a third time after an additional time I/.alpha.'. If this third measurement produces a value .alpha." greater than 1, then an indication is generated that the endpoint has been reached. Thus, an endpoint indication is generated only if the ratio of the slope of the averaged signal to the reference slope exceeds unity in three successive box measurements.
Although this averaging procedure can extract the endpoint signal, there are a number of problems with this approach. The averaging process adds some complexity to the endpoint detection process. More significantly, the averaging process tends to wash out the endpoint signal and produces a delay between the time when the endpoint occurs and the time when the endpoint is detected. These problems are illustrated in FIG. 5.
FIG. 5 illustrates these problems for a situation in which the averaged signal is averaged over an averaging time T.sub.a and in which the transition that occurs in this input signal 52 at the endpoint takes place over an interval of duration T.sub.t. The resulting transition in averaged signal 51 occurs over an interval of duration T.sub.a +T.sub.t. Thus, the slope of this transition is decreased by a factor T.sub.t /(T.sub.t +T.sub.a). Typical process times are from 30-120 seconds and typical transition times are about 5%-8% of the process time. Thus, typical values of T.sub.t are 1.5-9.6 seconds. For example, if the transition in input signal 52 takes a time T.sub.t =4 seconds and the averaging time T.sub.a is 40 seconds, then the slope is reduced by a factor of 1/11.
When the slope of a transition in the averaged signal is small, low levels of noise and the remaining semiperiodic component of the averaged signal will significantly affect the slope of the curve within the transition interval. This increased sensitivity of low slope transitions strongly counters the benefits of the averaging process.
The averaging process also delays the detection of the endpoint. If the midpoint of the transition in signal 52 is taken as the endpoint, then the detected endpoint occurs in the middle of the transition in signal 51. The detected endpoint thus occurs an interval T.sub.a /2 later than the actual endpoint. Thus, the averaging process introduces a sizable delay between the occurrence of the endpoint and the detection of this occurrence. Especially in wafer process steps that are completed in a time comparable to or smaller than the averaging time T.sub.t, this delay will have a significant effect on throughput and on overprocessing of the wafer. For example, for an etch process taking 30 seconds and an averaging time of 40 seconds, there will be 67% overprocessing. This amount of overprocessing is particularly critical in plasma etch processes because the plasma often etches other layers at significant rates. Therefore, it is important to eliminate or at least significantly reduce the amount of averaging that is required to pull the endpoint signal out of the input signal S to endpoint system 18.